Plasmonic Resonance of Active Dimer Systems in the Presence of Optical Binding Forces

摘要

Optical properties of metal nanoparticles are related to the collective oscillations of conduction electrons. In metals, these electrons are loosely bound with the lattice core and therefore oscillate freely with the incident electromagnetic wave. Such oscillations are popularly known as plasmonic resonance. The term nanoparticle, in general, refers to small cluster of atoms of size ranging from one nanometer to few hundred nanometers. Plasmonic resonance in metal spherical and core-shell nanoparticles yields a number of different properties and remarkably such resonances are tunable with both the geometrical parameters of the particles involved and also with the background. When more than one metal nanoparticles are located at close proximity to each other, can influence each others optical responses. In this case, their responses can be tuned by, along with many other means, varying the interparticle separations. This work attempts the theoretical design of a tunable surface with two metallic nanoparticles where the interparticle separation is varied with the aid of optical binding forces. When such a tunable surface is subjected to a narrow band light incidence, it can shift the central frequency of the incident light. The amount of such frequency shifts can be controlled with their separation distances.;In this work, that separation distance is tuned with the optical binding force. The optical binding phenomenon involves more than one particles. It may be defined as the stable spatial reconfiguration of nanoparticles due to light illumination and a simultaneous redistribution of incident light by the particles. This thesis work begins with a Maxwell Stress Tensor based formulation of the optical binding force between two Rayleigh particles upon a unit amplitude plane wave illumination. Next, this framework was extended to a narrow band light by decomposing the narrow band light into a large number of plane wave components with different angular velocities.;The total optical binding force increases with the increasing bandwidth of the incident light and the central frequency dictate the nature of the binding force (attractive or repulsive). Afterwards, a tunable surface comprised of a silver nanoparticle dimer is demonstrated. It was found that with the increase of the interparticle separation, the response of the surface shifts towards the higher wavelengths (red shift). On the other hand, when the the particles come close towards one another, the output band widens compared to that of the former, along a red shift of the response.